1. Field of the Invention
The present invention relates to a trolley, particularly a monorail trolley having a short structural height. The trolley has travel wheels which travel on the lower flange of a rail. The trolley further includes a lifting mechanism arranged on a support frame and a drivable friction wheel which can be pressed through a spring element against the lower flange of the rail.
2. Description of the Related Art
U.S. Pat. No. 2,617,365 discloses a trolley of the above-described type which includes a carriage which travels on a rail and on which is arranged a lifting mechanism. The carriage is connected through a drag link to a drivable travel unit which is provided with two wheels traveling on the lower flange of the I-shaped rail. The wheels are each connected through gear wheels for driving the wheels to a friction wheel, wherein the friction wheel rests against the bottom side of the flange, slightly offset relative to the wheels as seen in travel direction. The friction wheels are arranged on a shaft and are driven by a drive motor through a chain. The axes or the shafts of the wheels and the friction wheels are fastened to a support plate. The drive motor is arranged on the support plate in such a way that the pressing force of the wheels and the friction wheels against the lower flange is increased through the support plate, which acts as a lever and the frictional engagement is thus reinforced.
It has been found to be disadvantageous that the connection of the carriage through the drag link with the travel unit significantly increases the structural length of the trolley. In addition, although this type of construction does lead to an increase of the frictional engagement between the wheels and friction wheels and the rail, this increase depends on the weight of the drive motor and is very small. In addition, the increase is not adjustable and does not take into consideration the different load conditions of the trolley.
Also, German patent 1 259 070 discloses a trolley which carries a lifting mechanism, wherein the trolley can travel by means of wheels on the lower flange of an I-shaped rail. The trolley is driven by means of a driven friction wheel which presses from below against the lower flange. The pressing force of the frictional wheel is applied by a pre-tensioned spring and is adjustable.
This type of drive of the trolley is useful for many types of applications because the pressing force of the friction wheel is adjustable. However, also in this case, the pressing force of the friction wheel does not automatically adapt to the load applied to the trolley. Moreover, in order to be able to transmit the necessary drive forces when the wheels are not driven, it is necessary to have a friction wheel which is of rubber or plastics materials having good frictional properties. This results in drive problems when the friction values decrease due to moisture.
German patent 491 527 discloses a trolley which travels on the lower flange of a rail and has a short structural height. This trolley consists essentially of a lifting mechanism, a support frame, a carriage with drive, and a counterweight. The lifting mechanism and the drive are fastened on opposite sides of the support frame which engages an I-shaped travel rail from below and approximately on the same vertical level with the rail. A counterweight is arranged on the side of the support frame located opposite the lifting mechanism, so that the trolley is balanced in such a way that the center of gravity is located below the middle of the rail.
While this trolley does have a low structural height, the eccentric arrangement of the lifting mechanism requires a counterweight for balancing the trolley. In addition to a disadvantageous increase of the weight, this also results in an increase of the structural size of the trolley, particularly when the counterweight is fastened to the support frame through lever arms.